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Visuals Make sure to include the location of your images; add a caption with this information This artist's drawing shows a supermassive black hole in the center of a galaxy. The black hole is surrounded by a cloud of material that is spiraling into it. Image Credit: NASA E/PO, Sonoma State University, Aurore Simonnet ||  This image of Sagittarius A in the center of the Milky Way galaxy was taken by the Chandra X-ray Observatory. Image Credit: NASA/CXC/MIT/F.K. Baganoff et al. ||  An artist's drawing shows a large stellar-mass black hole pulling gas away from a companion star. Image Credit: NASA E/PO, Sonoma State University, Aurore Simonnet ||
 * [[image:http://www.nasa.gov/images/content/279259main_BlackHole_4-xltn.jpg align="bottom" caption="An active galaxy with light shooting out of its center"]]
 * [[image:http://www.nasa.gov/images/content/391810main_image_1487_946-710.jpg width="403" height="328" align="bottom" caption="NGC 6240"]]

Black Holes Go 'Mano a Mano'
This image of NGC 6240 contains new X-ray data from Chandra (shown in red, orange, and yellow) that has been combined with an optical image from the Hubble Space Telescope originally released in 2008. In 2002, Chandra data led to the discovery of two merging black holes, which are a mere 3,000 light years apart. They are seen as the bright point-like sources in the middle of the image.Scientists think these black holes are in such close proximity because they are in the midst of spiraling toward each other -- a process that began about 30 million years ago. It is estimated that they holes will eventually drift together and merge into a larger black hole some tens or hundreds of millions of years from now.Finding and studying merging black holes has become a very active field of research in astrophysics. Since 2002, there has been intense interest in follow-up observations of NGC 6240, as well as a search for similar systems. Understanding what happens when these exotic objects interact with one another remains an intriguing question for scientists.The formation of multiple systems of supermassive black holes should be common in the universe, since many galaxies undergo collisions and mergers with other galaxies, most of which contain supermassive black holes. It is thought that pairs of massive black holes can explain some of the unusual behavior seen by rapidly growing supermassive black holes, such as the distortion and bending seen in the powerful jets they produce. Also, pairs of massive black holes in the process of merging are expected to be the most powerful sources of gravitational waves in the Universe.//Image Credits: X-ray: NASA/CXC/MIT/C.Canizares, M.Nowak; Optical: NASA/STScI// || ||   ||

**Works Cited** **Sources** : Include the source information for all of the magazine articles, reference sources (encyclopedias) and web site pages that were used to complete your project. The source information for encyclopedias may be found at the end or beginning of each entry in iCONN. When using periodicals, the publication information will be at the beginning or end of the article. This needs to be formatted for MLA standards. If it is not labeled 'Source Citation' it can be formatted appropriately by using EasyBib.com. You should use EasyBib for the web sites. The final Works Cited should be listed in alphabetical order by the first word of the source citation. "Milky Way." //Kids InfoBits Presents: Astronomy//. Gale, 2008. Reproduced in Kids InfoBits. Detroit: Gale, 2012. "The Milky Way." //WMAP's Universe//. NASA, 28 June 2010. Web. 06 Mar. 2012. . Vergano, Dan. "Galaxy Bracketed by Big Bubbles." //USA Today// 10 Nov. 2010: 05A. Web. 6 Mar. 2012.
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"What Is a Black Hole?" Heather R. Smith/NASA Educational Technology Services []
 * Your Source List:**

"Black hole." //The Columbia Encyclopedia//, 6th ed. Columbia University Press, 2000. Reproduced in Kids InfoBits. Detroit: Gale, 2012. []

"Black Holes." //World of Physics//. Gale, 2010. //Gale Science In Context//. Web. 9 Mar. 2012. Document URL []

"Black hole." //The Gale Encyclopedia of Science//. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. 4th ed. Detroit: Gale, 2009. //Gale Science In Context//. Web. 9 Mar. 2012. Document URL []

"Black hole." //U*X*L Encyclopedia of Science//. U*X*L, 2009. //Gale Science In Context//. Web. 12 Mar. 2012. Document URL []

"What Are The Parts of a Black Hole?" Science>Astronomy & Space WISTEME []

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 * Black Holes**
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 * What is a black hole?**

**Notes** "What Is a Black Hole?" Heather R. Smith/NASA Educational Technology Services []

A black hole is a region in space where the pulling force of gravity is so strong that light is not able to escape. The strong gravity occurs because matter has been pressed into a tiny space. This compression can take place at the end of a star's life. Some black holes are a result of dying stars.

Because no light can escape, black holes are invisible. However, space telescopes with special instruments can help find black holes. They can observe the behavior of material and stars that are very close to black holes.

Black holes can come in a range of sizes, but there are three main types of black holes. The black hole's **mass** and size determine what kind it is.
 * How Big Are Black Holes?**

The smallest ones are known as primordial black holes. Scientists believe this type of black hole is as small as a single atom but with the mass of a large mountain.

The most common type of medium-sized black holes is called "stellar." The mass of a stellar black hole can be up to 20 times greater than the mass of the sun and can fit inside a ball with a diameter of about 10 miles. Dozens of stellar mass black holes may exist within the Milky Way galaxy.

The largest black holes are called "supermassive." These black holes have masses greater than 1 million suns combined and would fit inside a ball with a diameter about the size of the solar system. Scientific evidence suggests that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a ball with a diameter about the size of the sun.

Primordial black holes are thought to have formed in the early universe, soon after the big bang.
 * How Do Black Holes Form?**

Stellar black holes form when the center of a very massive star collapses in upon itself. This collapse also causes a supernova, or an exploding star, that blasts part of the star into space.

Scientists think supermassive black holes formed at the same time as the galaxy they are in. The size of the supermassive black hole is related to the size and mass of the galaxy it is in.


 * If Black Holes Are "Black," How Do Scientists Know They Are There?**

A black hole can not be seen because of the strong gravity that is pulling all of the light into the black hole's center. However, scientists can see the effects of its strong gravity on the stars and gases around it. If a star is orbiting a certain point in space, scientists can study the star's motion to find out if it is orbiting a black hole.

When a black hole and a star are orbiting close together, high-energy light is produced. Scientific instruments can see this high-energy light.

A black hole's gravity can sometimes be strong enough to pull off the outer gases of the star and grow a disk around itself called the accretion disk. As gas from the accretion disk spirals into the black hole, the gas heats to very high temperatures and releases X-ray light in all directions. NASA telescopes measure the X-ray light. Astronomers use this information to learn more about the properties of a black hole.


 * Could a Black Hole Destroy Earth?**

Black holes do not wander around the universe, randomly swallowing worlds. They follow the laws of gravity just like other objects in space. The orbit of a black hole would have to be very close to the solar system to affect Earth, which is not likely.

If a black hole with the same mass as the sun were to replace the sun, Earth would not fall in. The black hole with the same mass as the sun would keep the same gravity as the sun. The planets would still orbit the black hole as they orbit the sun now.

_

"Black hole." //The Columbia Encyclopedia//, 6th ed. Columbia University Press, 2000. Reproduced in Kids InfoBits. Detroit: Gale, 2012. []

Black hole, in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping. The term was first used in reference to a star in the last phases of gravitational collapse (the final stage in the life history of certain stars; see stellar evolution) by the American physicist John A. Wheeler. Gravitational collapse begins when a star has depleted its steady sources of nuclear energy and can no longer produce the expansive force, a result of normal gas pressure, that supports the star against the compressive force of its own gravitation. As the star shrinks in size (and increases in density), it may assume one of several forms depending upon its mass. A less massive star may become a white dwarf, while a more massive one would become a supernova. If the mass is less than three times that of the sun, it will then form a neutron star. However, if the final mass of the remaining stellar core is more than three solar masses, as shown by the American physicists J. Robert Oppenheimer and Hartland S. Snyder in 1939, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, a point called the "singularity."At the point of singularity the effects of Einstein's general theory of relativity become paramount. According to this theory, space becomes curved in the vicinity of matter; the greater the concentration of matter, the greater the curvature. When the star (or supernova remnant) shrinks below a certain size determined by its mass, the extreme curvature of space seals off contact with the outside world. The place beyond which no radiation can escape is called the event horizon, and its radius is called the Schwarzschild radius after the German astronomer Karl Schwarzschild, who in 1916 postulated the existence of collapsed celestial objects that emit no radiation. For a star with a mass equal to that of the sun, this limit is a radius of only 0.9 mi (1.5 km). Even light cannot escape the black hole but is turned back by the enormous pull of gravitation.It is now believed that the origin of some black holes is nonstellar. Some astrophysicists suggest that immense volumes of interstellar matter can collect and collapse into supermassive black holes, such as are found at the center of some galaxies. The British physicist Stephen Hawking has postulated still another kind of nonstellar black hole. Called a primordial, or mini, black hole, it would have been created during the "big bang," in which the universe was created (see cosmology). Unlike stellar black holes, primordial black holes create and emit elementary particles, called Hawking radiation, until they exhaust their energy and expire. It has also been suggested that the formation of black holes may be associated with intense gamma ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting electromagnetic radiation in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance.Because light and other forms of energy and matter are permanently trapped inside a black hole, it can never be observed directly. However, a black hole can be detected by the effect of its gravitational field on nearby objects (e.g., if it is orbited by a visible star), during the collapse while it was forming, or by the X rays and radio frequency signals emitted by rapidly swirling matter being pulled into the black hole. A small number of possible black holes have been detected. The first discovered (1971) was Cygnus X-1, an X-ray source in the constellation Cygnus. In 1994 astronomers employing the Hubble Space Telescope announced that they had found conclusive evidence of a supermassive black hole in the M87 galaxy in the constellation Virgo. The first evidence (2002) of a binary black hole, two supermassive black holes circling one another, was detected in images from the orbiting Chandra X-ray Observatory. Located in the galaxy NGC6240, the pair are 3,000 light years apart, travel around each other at a speed of about 22,000 mph (35,415 km/hr), and have the mass of 100 million suns each. As the distance between them shrinks over 100 million years, the circling speed will increase until it approaches the speed of light, about 671 million mph (1080 million km/hr). The black holes will then collide spectacularly, spewing radiation and gravitational waves across the universe.  _

"Black Holes." //World of Physics//. Gale, 2010. //Gale Science In Context//. Web. 9 Mar. 2012. Document URL http://ic.galegroup.com/ic/scic/ReferenceDetailsPage/ReferenceDetailsWindow?displayGroupName=Reference&disableHighlighting=true&action=e&windowstate=normal&catId=GALE%7C00000000MV92&documentId=GALE%7CCV2434500043&mode=view&userGroupName=s0002&jsid=f085da76d027972f33c710acfb47c265

A black hole is the area of space surrounding a infinitesimally small point of infinitely dense matter (a singularity)--literally the mass of a large sun, collapsed into immeasurably small space). The gravitational field produced by the singularity is so intense that light cannot escape the boundary between the black hole and surrounding space. The modern theoretical prediction of black holes came as a spectacular consequence of German-American physicist Albert Einstein's (1879-1955) general theory of relativity. Einstein's theory predicted that massive stars ultimately collapse into an infinitesimally small space with a surrounding gravitational field so intense that light can not escape. In 1969, American physicist John A. Wheeler popularized the term "black hole." to describe this area of space. During the later half of the twentieth century, the study and discovery of black holes became one of the preeminent quests of modern astronomy. In the 1930s, Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910-1995) mathematically proved that black holes were the remains of massive stars and fully articulated the evolution of stars into supernova, white dwarfs, neutron stars , or black holes. Before the intervention of World War II, American physicist J. Robert Oppenheimer (1904-1967)--who ultimately supervised project Trinity, the making of the first atomic bombs--made detailed calculations reconciling Chandrasekhar's predictions with general relativity theory. In the late 1960s, English mathematician and physicist Stephen Hawking (1942-), along with English mathematician Roger Penrose (1931-), drew from both quantum and relativity theory to show that within a black hole there must exist a singularity (also defined as a geometric point without definable space) of and therefore of infinite density. Penrose advanced a scientific and philosophic concept known as the "law of cosmic censorship," an assertion that regions around the singularity (i.e., black holes) are regions of space cut off from direct human observation. In 1963, New Zealand mathematician and astrophysicist Roy Patrick Kerr (1934-) used Einstein's field equations to predict the existence of rotating black holes. Essentially, throughout the life of a star a tug-of-war exists between the compressing force of the star's own gravity and the expanding pressures generated by nuclear reactions at its core. After cycles of swelling and contraction associated with the burning of progressively heavier nuclear fuels, the star eventually runs out of usable nuclear fuel. The spent star then contracts under the pull of it own gravity. Modern understandings of astrophysics allow three possible fates for such a collapsing star. There are two types of black holes that are studied by astronomers: stellar-mass black holes (that result when stars about 10 times the size of the Sun end their life) and supermassive black holes (believed to exist at the center of large galaxies). The particular fate for any star is determined by the mass of the star left after blowing away its outer layers during its death spasms. A star less than 1.44 times the mass of the Sun --termed the Chandrasekhar limit --collapses until the pressure in the increasing compacted electron clouds exerts enough pressure to balance the collapsing gravitational force. Such stars become white dwarfs contracted to a radius of only a few thousand miles (roughly the size of a planet). Since the majority of stars in the visible universe are low-mass stars, this is the fate of most stars. If the star retains between 1.4 and roughly three times the mass of the Sun, the pressure of the electron clouds is insufficient to stop the gravitational collapse. In such stars, contraction continues to produce a neutron star that is only a few miles in radius. Within a neutron star, the repulsion forces from the compressed atomic nuclei balance the crushing force of gravity. With more massive stars, however, there is no known force in the universe that can withstand the gravitational collapse. Such extraordinary stars will continue their collapse to form a singularity--a star collapsed to a point of infinite density. As such a star collapses its gravitational field warps space-time so intensely that not even nearby light can escape, thus forming a stellar-mass black hole. The event horizon is the region marking the outer boundary of the black hole. The distance between the singularity and the event horizon is termed the Schwarzschild radius, named after the German astronomer Karl Schwarzschild (1873-1916). Inside the event horizon, the gravitational attraction of the singularity is so strong that the required escape velocity is greater than the speed of light. As a consequence, because no object can exceed the speed of light, neither light nor matter can escape from the region within the event horizon. No information generated within the black hole can escape, making the event horizon an important observational boundary. Essentially, all astronomers can know with any certainty regarding the processes of the singularity are the external gravitational effects exerted by its tremendous mass. To identify systems that are good black hole candidates, astronomers identify regions in space where a star orbits around an unseen companion and there is strong electromagnetic radiation from an unidentified source near the center of rotation. An accretion disk forms as matter from the companion star accelerates toward the event horizon of the black hole. As the matter in the accretion disk spirals toward the black hole, it is heated to very high temperatures and emits strong, highly energetic electromagnetic radiation (such as x-rays and gamma rays). Some astronomers assert that such a mechanism, working on a galactic scale, may account for the phenomena associated with quasars. Astronomers continue to find new binary star systems where they believe stellar-mass black holes exist. In 1974, British astronomer Sir Martin Rees proposed that supermassive black holes might exist in the middle of some galaxies. He believed these black holes, consisting of millions of solar masses, could explain the enormous amount of radiation observed at the nuclei of galaxies. In 1994, the Hubble Space Telescope provided arguably conclusive evidence for the existence of a supermassive black hole located at the center of the M87 galaxy. Similar evidence indicates that a black hole also lies at the center of the Milky Way galaxy. Recent evidence from the Hubble Space Telescope indicates intermediate-sized black holes (consisting of thousands of solar masses) exist in the cores of globular star clusters that orbit the Milky Way and other galaxies. Although the events that occur inside a black hole remain an enigma, some physicists have attempted to speculate about the nature of time dilations and contractions near the event horizon. Hawking radiation --involving massless virtual particles and particle-antiparticle pairs, for example--may explain mass and radiation leakage from black holes. Astronomers also suggest that, in the early universe, black holes could have formed as a consequence of the collection and collapse of large volumes of interstellar gas. In addition to exerting intense gravitational attraction, black holes influence events far from their actual locations. For example, Earth is constantly being struck by cosmic rays, which are fast-moving subatomic particles and atomic nuclei from space. Some cosmic rays are so energetic that they have astonished scientists; a few carry as much energy as a fast tennis ball (over 100x1018 electron volts). The source of these ultrahigh-energy cosmic rays has been a mystery since they were first observed in 1962, partly because only a few strike each square kilometer of Earth's surface every thousand years. In 2007, a team of astronomers operating a 1,160 square-mile (3,000 square kilometer) array of detectors in Argentina, from the Pierre Auger Southern Observatory, traced these particles to their likely source: active galactic nuclei, galactic centers radiating large amounts of energy. Supermassive black holes, from a million to ten billion times more massive than Earth's sun, are found at the centers of such galaxies. Matter that falls into a black hole does not come out again, but particles that are accelerated near a supermassive black hole by its gravity can be ejected at speeds very close to the speed of light. A few of these particles strike Earth as ultrahigh-energy cosmic rays. The existence of black holes opens the possibility for the existence of another stunning concept known as wormholes (also termed Einstein-Rosen bridges). A wormhole is a mathematical solution to Einstein's relativistic equation for gravity in which two parts of space-time may be joined together. There is no hard evidence for the existence of wormholes, but they remain as an intriguing theoretical possibility.

_ Source Citation: "Black hole." //The Gale Encyclopedia of Science//. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. 4th ed. Detroit: Gale, 2009. //Gale Science In Context//. Web. 9 Mar. 2012. Document URL []

A sufficiently intense gravitational field can prevent the escape of both matter and light. Such regions of space are known as black holes. Formed from the destruction of massive stars, black holes are regions of high gravity surrounding a singularity, the remnant of a star of great mass reduced to a point of infinitesimally small space. Photons, traveling at the speed of light, do not have sufficient escape velocity to penetrate the event horizon surrounding the black hole. The maximum intensity of a spherical object's gravitational field is a function both of the amount of matter it contains and of its volume. The more matter is contained in an object and the smaller its volume--in other words, the higher its density--the more intense the gravitational field at its surface will be. If the Earth were compacted so that it had the same mass, but half its present radius, the force of gravity at its surface would be four times as great as it is now; if it were compacted further, a density would eventually be reached at which its constituent subatomic particles would be unable to support their own weight and would collapse to a state of (theoretically) infinite density, producing a black hole. Black holes can (and some do) contain very large amounts of matter--millions or billions of times the mass of the Sun--but may be formed by even a small amount of matter sufficiently compressed. The idea of black holes is not new. French mathematician Pierre Simon Laplace (1749-1847) reasoned in 1795 that if the corpuscular theory of light proposed by English physicist Isaac Newton (1642-1727) were correct, there could exist massive objects from which light could not escape. The theory of general relativity, put forward by German physicist Albert Einstein (1879-1959) in 1915 and today basic to physicists' understanding of the universe, also predicts the existence of black holes, though using rather different reasoning. In recent decades, much observational evidence has been gathered to support the existence of black holes. There is no debate among astronomers today about whether black holes exist, only regarding their precise properties. The first identified black hole candidate was found associated with the star Cygnus and designated Cygnus X-1 (an intense x-ray source). Following initial discovery of the x-ray emissions in 1965, subsequent observations in 1973 by astronomers using the Copernicus satellite provided evidence that Cygnus and Cygnus X-1 was actually a binary star system (a binary star is a pair of stars in a single system that orbit each other, bound together by their mutual gravities). The source of the intense x-ray emissions was a stellar-sized black hole, the unseen companion of Cygnus. In 1975, much more definitive evidence of the existence of black holes came with the discovery of the x-ray source A0620-00, and its optical counterpart Nova Mon 1975 based on observations made via the Ariel V satellite.

The event horizon
According to general relativity, the path taken by a beam of light is the shortest distance between two points; such a path is called a geodesic. Furthermore, gravity warps space, bending geodesics; the stronger a gravitational field is in a certain region, the more bent the geodesics are in that region. Within a certain radius of a black hole, all geodesics are so warped that a photon of light cannot escape to another part of the universe. Essentially, there are no straight lines connecting any point that is within a certain radius of a black hole (which, in theory, means there is no dimension) to any point that is farther away. The spherical surface defined by this radius is termed the event horizon of the black hole because events inside the event horizon can have no effect on events outside it. Whatever is inside the event horizon is sealed off forever from the space-time of outside observers. The event horizon, thus, imposes a form of censorship on the makeup of a black hole; the only properties of a black hole that can be ascertained from the outside are its mass, net charge, and rate of spin. No internal time-dependent processes can be detected in the external environment, since that would involve sending signals from inside the black hole to the outside--which is impossible, for not even light can escape. This censorship, or inability to produce information from within the black hole, is what is responsible for the fewness of a black hole's measurable properties: mass, spin, and charge. Although there are complications in defining the size of a black hole--due to the fact that the everyday concept of size assumes Euclidean three-dimensional space and such space does not exist even approximately in the near vicinity of a black hole--one can uniquely specify a black hole's circumference and thus its radius as the circumference divided by 2π, where π is a mathematical constant equal approximately to 3.1459. This value is known as the Schwarzschild radius (Rs) after German astronomer Karl Schwarzschild (1873-1916), who first defined it as Rs =2 GM / c 2 where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light. Rs, however, cannot be interpreted as the radius of a Euclidean sphere--that is, as the distance from a spherical surface (the event horizon) to its center (the black hole). As mentioned above, the geometry of space-time in the interior of the black hole is so warped that Euclidean notions of distance no longer apply. Nevertheless, Rs does provide a measure of the space around a black hole of mass M. Rs for an object having the mass of the Sun is about 3 kilometers (km). Thus, in order to turn the Sun into a black hole, one would have to compress it from a sphere with a radius of 696,000 km to a sphere with a radius of just 3 km. Squeezing any mass into a volume dictated by its Schwarzschild radius presents a serious assembly problem. In fact, the only processes that might lead to the formation of a sizable black hole are the explosive death of a moderately massive star or the formation of a supermassive star by sheer accumulation. Physicists also speculate that extremely small black holes might be created by the collision of subatomic particles at high energies. In fact, they estimate that as many as 100 subatomic-size black holes may be produced in the atmosphere of the Earth every year by cosmic rays. The European Laboratory for Particle Physics (CERN, for Conseil Européen pour la Recherche Nucléaire) hopes to produce such microscopic black holes on demand in its new Large Hadron Collider, due to begin operation in November 2007. As of 2006, the artificial formation of mini-black holes in particle accelerators has been reported, but their creation has not yet been confirmed. Very small black holes are predicted by theory to be short-lived, however, due to a quantum phenomenon termed evaporation. Only large black holes are long-lived enough to have cosmic effects--to swallow millions of Suns' worth of mass, to squeeze sufficient energy from the matter approaching their event horizons to outshine entire galaxies, to organize the orbits of billions of stars into well-defined galaxies, and so forth. Large black holes are thought to form primarily from exploding stars or by direct gravitational accumulation of large quantities of matter. A black hole may be produced by an exploding star (nova) as follows: An older star eventually exhausts the nuclear fuel that enables it to produce energy at its core, thus supporting its own weight (and shining steadily for many millions of years). It then begins a rapid collapse. The crushing pressure of the collapsing matter may be sufficient to form a black hole with the mass of several times that of the Sun. Such black holes would have Schwarzschild radii of several to a few tens of kilometers. Considering the amount of mass filling that space, such objects are truly tiny.

Detection of black holes
In their near vicinity, black holes produce bizarre effects; from a distance, however, they are well-behaved. If one were to replace the Sun with a black hole of the same mass as the Sun, there would be a region of space a few kilometers in size, located where the center of the sun currently resides, in which space would be extremely warped. The gravitational field of this object, measured at the distance of Earth, would be exactly that of the present-day Sun. Earth and planets would continue in their orbits and the solar system would continue much as it does today--only in the dark. Normally, an observer must get within a few Schwarzschild radii in order to feel the distinctive effects of the black hole. Indeed, one of the observational tests for the presence of a black hole in binary (two-body) systems is to look for the characteristic radiations of matter being heated as it is squeezed during its final plunge toward the black hole's event horizon. Such matter will emit fluctuating x rays because of being squeezed. The rate of fluctuation is tied to the size of the emitting region. Astronomers find that in such systems the x rays come from a volume of space only a few kilometers in diameter. In several instances, analysis of the orbital motions in a binary star system with only one visible member (a conventionally shining star) indicates that the dark, unseen member of the binary system is much more massive than the Sun. A dark stellar component more massive than the Sun confined to a volume smaller than a few kilometers is a prime candidate for a black hole. There is at least one other situation in which astronomers suspect the existence of a black hole. Because a black hole that is not actively swallowing large amounts of matter does not radiate significantly, astronomers must detect it indirectly, through the effect of its gravitational field on neighboring objects. In the centers of many galaxies, the stars, gas, and dust of the galaxy are moving at very high speeds, suggesting they are orbiting some very massive, comparatively small object. If the object was a tightly packed collection of massive stars, it would shine so brightly as to dominate the light from the galactic center. The absence of light from the massive, central object suggests it is a black hole. Recent astronomical observations have confirmed many galaxies, including the Milky Way, have supermassive black holes at their centers. Some scientists believe that all galaxies may be organized around such black holes; after the Big Bang, supermassive black holes may have formed first, then gathered the galaxies around them. In one galaxy, the Hubble Space Telescope (HST) has photographed a spiraling disk of matter that appears to be accreting onto a central massive dark object that is likely to be a black hole. Recently a large team of astronomers reported the results of a worldwide study involving the HST, the International Ultraviolet Explorer (an astronomical research satellite), and many ground-based telescopes. Instruments were able to detect light that was emitted by the accreting matter as it spiraled into the black hole that was subsequently absorbed and re-emitted by the orbiting clouds just a few light-days away from the central source. Mass estimates of the central source determined from the motion of these clouds suggest that the object has a mass of at least several million times the mass of the Sun. So much material contained in a volume of space no larger than a few light-days in diameter provides some of the clearest evidence yet for the existence of a black hole at the center of any galaxy. In another study with the HST and a ground-based telescope in Hawaii, scientists were able to observe a black hole in a two-star system in the constellation Cygnus. This black hole is sucking material from its companion star in a swirling disk of material and hot gases, swallowing nearly 100 times as much energy as it radiates. The material being pulled in toward the black hole stores its energy as heat until the critical moment. The observations show gas at temperatures of over a million degrees falling toward the event horizon of the black hole. Until very recently, scientists only had convincing evidence of two classes of black holes. The first class, stellar mass black holes, form from the remains of collapsed stars that have, at most, 10 times the mass of the Sun. The second type, supermassive black holes, are thought to have formed when the universe was very young. It is also thought that they are common, existing at the core of every galaxy in the universe. These gigantic black holes have masses up to that of a billion Suns. In 2000, astrophysicists from Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, announced they had convincing evidence for a new class of intermediate-mass black holes. Using observations from the Chandra X-ray Observatory of a galaxy about 12 million light-years from Earth, the scientists theorized the existence of a black hole in that galaxy with a mass of a least 500 Suns. In 2009--using data gathered by the European Space Agency's XMM-Newton x-ray space telescope--astrophysicists at the Centre d'Etude Spatiale des Rayonnements in France published a report in the scientific journal Nature of a more definitive detection of an intermediate-mass black hole approximately 500 times the mass of the Sun and approximately 290 million light years from Earth. The discovery provides a key stepping stone in validating the hypothesis that massive black holes may form from aggregations of smaller or intermediate-mass black holes. Recent evidence from the Hubble Space Telescope indicates intermediate-sized black holes (consisting of thousands of solar masses) exist in the cores of globular star clusters that orbit the Milky Way and other galaxies.

Centerpiece of the galaxy
The concept of massive black holes at the centers of some galaxies is supported by theoretical investigations of the formation of very massive stars. Stars of more than about a hundred times (and less than about a million times) the mass of the Sun cannot form because they will explode from nuclear energy released during their contraction before the star can shrink enough for its self-gravity to hold it together. However, if a collapsing cloud of interstellar material contains more than about a million times the mass of the Sun, the collapse will occur so fast that the nuclear processes initiated by the collapse will not be able to stop the collapse. The collapse will continue unrestrained until the object forms a black hole. Such objects appear to be required to understand the observed behavior of the material in the center of some galaxies. Indeed, it is now certain that black holes reside at the centers of many normal galaxies, including the Milky Way. Evidence comes from the motion of gas clouds near the galactic center and from the detection of x-ray bursts from the galactic center, such as would typically be produced by a supermassive black hole swallowing matter. Earth is constantly being struck by cosmic rays, which are fast-moving subatomic particles and atomic nuclei from space. Some cosmic rays are so energetic that they have astonished scientists; a few carry as much energy as a fast tennis ball (over 100x1018 electron volts). The source of these ultrahigh-energy cosmic rays has been a mystery since they were first observed in 1962, partly because only a few strike each square kilometer of Earth's surface every thousand years. In 2007, a team of astronomers operating a 1,160 square-mile (3,000 square-kilometer) array of detectors in Argentina, from the Pierre Auger Observatory, traced these particles to their likely source: active galactic nuclei, galactic centers radiating large amounts of energy. Supermassive black holes, from a million to ten billion times more massive than Earth's Sun, are found at the centers of such galaxies. Matter that falls into a black hole does not come out again, but particles that are accelerated near a supermassive black hole by its gravity can be ejected at speeds very close to the speed of light. A few of these particles strike Earth as ultrahigh-energy cosmic rays.

Quantum physics and black holes
All that has been said so far involves black holes as described by the general theory of relativity (as written by Einstein). However, in the realm of the very small, quantum mechanics has proved to be the proper theory to describe the physical world. To date, no one has successfully combined general relativity with quantum mechanics to produce a fully consistent theory of quantum gravity. However, in 1974, British physicist Stephen Hawking (1942-) suggested that quantum principles showed that a black hole should radiate energy like a perfect radiator having a temperature inversely proportional to its mass. This radiation--termed Hawking radiation--does not come about by the conventional departure of photons from the black hole's surface, which is impossible, but as a result of certain effects predicted by quantum physics. While the amount of radiation for any astrophysical black hole is very small (e.g., the radiation temperature for a black hole with the mass of the Sun would be 10-7K [Kelvin]), the suggestion that loss of energy from a black hole was possible at all was revolutionary. It suggested a link between quantum theory and general relativity. The suggestion has spawned a host of new ideas expanding the relationship between the two theories. It is the ability of a black hole to lose mass via Hawking radiation (i.e., to evaporate) that prevents microscopic black holes, such as those that physicists hope to produce at CERN, from swallowing up the Earth. These black holes evaporate faster than they can grow.

"Black hole." //U*X*L Encyclopedia of Science//. U*X*L, 2009. //Gale Science In Context//. Web. 12 Mar. 2012. Document URL http://ic.galegroup.com/ic/scic/ReferenceDetailsPage/ReferenceDetailsWindow?displayGroupName=Reference&disableHighlighting=true&prodId=SCIC&action=e&windowstate=normal&catId=&documentId=GALE%7CCV2644300174&mode=view&userGroupName=s0002&jsid=b33719915ae4d55e23b26fd321c8ce38

A black hole is all that remains of a massive star that has used up its nuclear fuel. Lacking energy to combat the force of its own gravity, the star compresses or shrinks in size to a single dimensionless point, called a singularity. At this point, pressure and density are infinitely large. Any object or even light that gets too close to a black hole is pulled in and trapped forever. Black holes, so named by American theoretical physicist John Archibald Wheeler (1911-2008) in 1969, are impossible to observe directly, but may be proven to exist by the effects the black hole's intense gravity on other objects in the universe. English natural philosopher and geologist John Michell (1724-1793) and French mathematician and astronomer Pierre-Simon Laplace (1749-1827) first developed the idea of black holes in the eighteenth century. They theorized that if a celestial body were large enough and dense enough, it would exhibit so much gravity that nothing could escape its pull. This idea can be explained by looking at the effects of gravity on known objects. To break free of Earth's gravity, a projectile has to be given an initial speed of at least 7 miles (11 kilometers) per second. To escape a larger planet like Jupiter, it would have to travel at 37 miles (60 kilometers) per second. And to escape the Sun, it would have to travel at 380 miles (611 kilometers) per second. A large and dense enough object could require the spaceship to go faster than the speed of light, 186,000 miles (299,000 kilometers) per second. However, because nothing can travel faster than the speed of light, nothing would be able to escape the gravity of such an object. Black holes, indeed, are such objects.

Black hole formation
Once a star's nuclear fuel is spent, it will collapse. Without the force of nuclear fusion pushing outward from its core to balance its immense gravity, a star will fall into itself. Average-sized stars, like the Sun, shrink to become white dwarfs (small, extremely dense stars having low brightness) about the size of Earth. Stars up to three times the mass of the Sun explode to produce a supernova. Any remaining matter of such stars ends up as densely packed neutron stars or, pulsars (rapidly rotating stars that emit varying radio waves at precise intervals), or magnetars (super-magnetized stars). Stars more than three times the mass of the Sun explode in a supernova and then, in theory, collapse to form a black hole. When a giant star collapses, its remaining mass becomes so concentrated that it shrinks to an infinitely small size and its gravity becomes completely overpowering. According to German-American physicist Albert Einstein's (1879-1955) general theory of relativity, space becomes curved near objects or matter; the more concentrated or dense that matter is, the more space is curved around it. When a black hole forms, space curves so completely around it that only a small opening to the rest of normal space remains. The surface of this opening is called the event horizon, a theorized point of no-return. Any matter that crosses the event horizon is drawn in by the black hole's gravity and cannot escape, vanishing across the boundary like water disappearing down a drain.

Black hole evidence
Black holes cannot be seen because matter, light, and other forms of energy do not escape from them. They can possibly be detected, however, by their effect on visible objects around them. Scientists argue that as gaseous matter swirls in a whirlpool before plunging into a black hole, that heated matter emits fluctuating x rays. Discovery of such a condition in space, therefore, may indicate the existence of a black hole near the source of those x rays. The first identified black hole candidate was found associated with the star Cygnus and designated Cygnus X-1 (an intense x-ray source). Following initial discovery of the x-ray emissions in 1965, subsequent observations in 1973 by astronomers using the Copernicus satellite provided evidence that Cygnus and Cygnus X-1 was actually a binary star system (a binary star is a pair of stars in a single system that orbit each other, bound together by their mutual gravities) and that the unseen companion of Cygnus (and source of the intense x-ray emissions) was a stellar-sized black hole. In 1975, much more definitive evidence of the existence of black holes came with the discovery of the x-ray source A0620-00, and its visible binary partner, Nova Mon 1975. In the 1990s, the Hubble Space Telescope provided scientists with evidence that black holes probably exist in nearly all galaxies and in interstellar space between galaxies. The biggest black holes are those at the center of galaxies. In the giant galaxy M87, located in the constellation Virgo, swirling gases around a suspected massive black hole stretch a distance of 50 light-years, or 2,950 trillion miles (4,750 trillion kilometers). In early 2001, scientists announced that data from the Chandra X-ray Observatory and the Hubble Space Telescope (HST) provided the best direct evidence for the existence of the theorized event horizons. The two Earth-orbiting telescopes both surveyed matter surrounding suspected black holes that eventually disappeared from view. The Chandra telescope observed x-ray emissions that disappeared around six candidate black holes, while the HST observed pulses of ultraviolet light from clumps of hot gases as they faded and then disappeared around Cygnus X-1.

Types of black holes
Until very recently, scientists only had convincing evidence of two classes of black holes. The first class, stellar mass black holes, form from the remains of collapsed stars that have, at most, 10 times the mass of the Sun. The second type, supermassive black holes, are thought to have formed when the universe was very young. It is also thought that they are common, existing at the core of every galaxy in the universe. These gigantic black holes have masses up to that of a billion Suns. In 2000, astrophysicists from Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, announced they had convincing evidence for a new class of intermediate mass black holes. Using observations from the Chandra X-ray Observatory of a galaxy about 12 million light-years from Earth, the scientists theorized the existence of a black hole in that galaxy with a mass of a least 500 Suns. In 2009--using data gathered by the European Space Agency's XMM-Newton x-ray space telescope--astrophysicists at the Centre d'Etude Spatiale des Rayonnements in France published a report in the scientific journal Nature of a more definitive detection of an intermediate-mass black hole approximately 500 times the mass of the Sun and approximately 290 million light years from Earth. The discovery provides a key stepping stone in validating the hypothesis that massive black holes may form from aggregations of smaller or intermediate-mass black holes.

Black hole impacts
Black holes create certain effects far from their actual locations. For example, Earth is constantly being struck by cosmic rays, which are fast-moving subatomic particles and atomic nuclei from space. Some cosmic rays are so energetic that they have astonished scientists; a few carry as much energy as a fast tennis ball (over 100x1018 electron volts). The source of these ultrahigh-energy cosmic rays has been a mystery since they were first observed in 1962, partly because only a few strike each square kilometer of Earth's surface every thousand years. In 2007, a team of astronomers operating a 1,160 square-mile (3,000 square kilometer) array of detectors in Argentina, from the Pierre Auger Observatory, traced these particles to their likely source: active galactic nuclei, galactic centers radiating large amounts of energy. Supermassive black holes, from a million to ten billion times more massive than Earth's Sun, are found at the centers of such galaxies. Matter that falls into a black hole does not come out again, but particles that are accelerated near a supermassive black hole by its gravity can be ejected at speeds very close to the speed of light. A few of these particles strike Earth as ultrahigh-energy cosmic rays and some may strike molecules of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) to cause genetic mutations that provide for new genes that drive biological evolution.

There are three parts to a simple black hole: Black holes can, in theory, come in any imaginable size. The size of a black hole depends on the amount of mass it contains. It's a very simple formula, especially if the black hole is not rotating. These 'non-rotating' black holes are called Schwarzschild Black Holes. ||
 * Black holes are objects that have such intense gravitational fields, they do not allow light to escape from them. They also make it impossible for anything that falls into them to escape, because to do so, they would have to travel at speeds faster than light. No forms of matter or energy can travel faster than the speed of light, so that is why black holes are so unusual!
 * **Event Horizon** - That's the part that we see from the outside. It looks like a black, spherical surface with a very sharp edge in space. The event horizon is where the force of gravity becomes so strong that even light is pulled into the black hole. Although the event horizon is part of a black hole, it is not a tangible object. If you were to fall into a black hole, it would be impossible for you to know when you hit the event horizon.
 * **Interior Space** - This is a complicated region where space and time can get horribly mangled, compressed, stretched, and otherwise a very bad place to travel through.
 * **Singularity** - That's the place that matter goes when it falls through the event horizon. It's located at the center of the black hole, and it has an enormous density. You will be crushed into quarks long before you get there!